Catfish study reveals importance of being ‘similar but different’

Two similar catfish species that share the same colour pattern: C. sp. C68 & C. maculifer A group of armoured catfishes abundant in small rivers and streams across South America are not all they appear- in fact communities are far more diverse and complex than previously suspected.

A new multidisciplinary study, reported in Nature (6.1.11), has enabled evolutionary biologists at Bangor University to establish for the first time that many Corydoras catfish that live together in the same rivers actually mimic each other’s colour patterns. Communities of catfishes consist of species that are not genetically closely related, but have, over a long time period, adopted one another’s colour patterns thereby communicating their unpalatability to predators and increasing their survival prospects.

However, the study also draws the conclusion that, intriguing as the shared colour patterns may be, it is not this evolutionary strategy that is the most important in terms of the ultimate survival and stability of mixed communities.

Three similar catfish species that share the same colour pattern C. tukano, C. sp. CW11, C. sp. C159 Using DNA analysis to investigate genetic relationships, stable isotopes to quantify diet and by quantifying differences in body shape and colour pattern of samples collected throughout South America, the research has established that 92% of co-existing mimetic species studied do not compete directly for food and belong to different genetic lineages.

Despite inhabiting the same habitat, adopting the same colour patterns and foraging together in shoals, these species do not compete for the same food. The scientists conclude that the ability of co-existing species to partition resources is the crucial element in maintaining long-term stability within communities.

“The species share the same appearance and the same habitat and very generally, the same food source. Current thinking would suggest that they would be in direct competition for resources, leading to ecological instability within the community as more successful competitors survive while others perish” said project leader Dr Martin Taylor from School of Biological Sciences, Bangor University, UK. Detailed investigation has however revealed that each species has a slightly different food source, thereby avoiding direct competition for food. Competition for resources is a powerful evolutionary force ensuring that coexisting species occupy a slightly different (trophic) niche, due to subtle differences in morphology that allow them to feed on different prey items.

The team’s results also suggest that the positive mutualistic benefits gained from adopting shared colour patterns do not outweigh the negative interactions of competition.

“Our research findings challenge recent theoretical and empirical work suggesting that mutualistic relationships are more important than competitive interactions, and can therefore sustain communities of species even when they compete for resources. Our results show that dietary differentiation and genetic relatedness are crucial factors that determine community structure and assembly to a greater extent than mutualistic interactions” said Markos Alexandrou PhD research student on the project.

The interdisciplinary methodology used in the research can be applied to other communities of co-existing species, while Corydoras catfishes provide a new model system for study of mimicry, competition, and mutualism. These findings may be bring us closer to an understanding of the critical factors and mechanisms responsible for the origin and maintenance of biodiversity in neotropical ecosystems.

Professor Graeme Ruxton, an evolutionary ecologist from the University of Glasgow said:

“Darwin himself saw mimicry between species as a really useful phenomenon for testing ideas about evolution, and it has fascinated evolutionary biologists ever since. This study will stimulate others to look more closely at the importance of competition for food between other mimicking species, and this may really help us to understand which species mimic and which do not. To me, this is the most exciting study in this field for quite some time, and very fitting that it should come just after the 200th anniversary of Darwin’s birth and the 150th anniversary of the publication of Origin of Species. “

The research was funded by the UK’s Natural Environment Research Council, and supported by UNESP, Brazil.